It is well-known in paint industry that the so-called plastic pigment has played an important role to reduce the cost of high quality water-borne paints without sacrificing performance by replacing part of titanium dioxide which has been mainly used as a white pigment. In general, the methods to increase the hiding power of paint film can be devided into two categories; one is based on the use of the open cell of microvoids and the other is based on the use of the closed cell of microvoids. The typical example of utilising microvoids of open cell type may be seen in the water-borne paints of low-cost and low quality, in which the pigment volume concentration of the paints are formulated above a critical pigment volume concentration by highly loading the so-called extender and hence the hiding power of the paint film are increased by the formation of interfaces between pigment and air instead of interfaces between pigment and binder. Regardless of the increased hiding efficiency this method suffers from inferior gloss and relatively poor resistances to water, soap solution and stain because of the presence of the connected large voids in the dried film.
In order to overcome the problems mentioned above, fine non-film forming emulsion polymer particles whose glass transition temperature is above 40.degree. C. and particle size is below 1.0 micron, e.g., polystyrene latexes, are formulated into the water-borne paint to reduce the size and quantity of continuous voids and to improve water resistance and washability with the help of the coalescing solvent added to partly sinter the non-film forming particles. However, this method also suffers from the fact that the hiding power of the paint films can not be maximized because of the lack of ability to scatter light rays by the emulsion polymer particles which has almost same reflectivity as that of film-forming latex binder.
Recently a number of approaches to incorporating microvoids of closed cell type into paint films have been suggested. One of these is the use of organic solvent which was embodied in U.S. Pat. No. 3,669,729. More specifically, the patent discloses the use of non-solvent entrapped film-forming latex binders as the main binder for latex paints whose dried film would be cellular form upon evaporation of the non-solvent and hence has an increased hiding power. However, despite of the improvement of hiding power, this method also suffers from such drawbacks as the complication of the process, poor storage stability, poor reproducibility of hiding power and the less environmental friendliness. A similar method was proposed in U.S. Pat. No. 3,637,431, in which non-film forming emulsion polymers emulsified with the organic solvent, immiscible with aqueous phase, were used to provide microvoids into the dried film, but it also suffered from the problems afore mentioned.
In order to overcome such problems a new method was proposed in recent years, in which microvoid-containing polymer particles were made in seperate process and blended into water-based coatings resulting in achieving more reliable opacifying method. Kershaw et. al, in U.S. Pat. No. 3,891,577 disclose preparation of vesiculated polymers by emulsion polymerization of a liquid medium which is obtained by the so-called double emulsification method, that is, emulsifying a water in oil emulsion, for example, the unsaturated polyester dissolved in styrene monomer containing water dispersed therein into water medium to become a water in oil in water emulsion and thus, by polymerizing such emulsion, obtaining polymer beads having 1 to 25 microns of diameter and several tiny water droplets inside of each bead. Whereas this material can play a better role as an opacifying pigment than polystrene solid particles because it has a hiding power itself, it is difficult for this material to have a contribution as the so-called spacer between TiO.sub.2 particles because this material is not able to be produced as submicron sized due to an inherent nature of emulsification method carried out by mechanical stirring, and also this material has a problem of settling during storage and in addition it can not afford to be applied in the field of glossy coating compositions.
In recent years in order to overcome the problems mentioned previously, a new preparation method has been developed where microvoid-containing polymer particles were prepared by sequentially emulsion polymerizing a core monomer system to be the alkali-swellable polymer core, polymerizing in the presence of the core polymer dispersion a shell monomer system, and neutralizing with ammonia or base so as to swell said core and form particles which, when dried, contain a single closed cell of void. The efficiency of the so-called core-shell emulsion polymerization may be depending on several factors such as monomer compositions of each stage of polymerization, glass transition temperature of the polymer, the kinds and concentration of surfactants, the kinds and concentration of initiators and polymerization temperature. For example, if the core polymer formed in the first stage is more hydrophilic than the shell polymer formed in the subsequent stage, the polymer particles finally obtained may have an inverse core-shell morphology or so-called confetti-like structure regardless of the order of monomer addition. To avoid the formation of undesirable particle morphology it has been suggested that a multi-stage emulsion polymerization process should be controlled in optimum way.
According to U.S. Pat. No. 427,836, after making a hydrophilio polymer containing carboxylic acid group, which can be expanding more than 2 times in volume when neutralizing with aqueous solution of volatile base, in first stage of emulsion polymerization, there has been used a method to form the shell on the surface of the hydrophilio core polymer with the polymeric material which is permeable to aqueous base solution by a thermal or redox emulsion polymerization process. In the said invention it was disclosed that the core polymer contains at least 5%, preferably at least 10%, by weight of acid monomers and the shell polymer contains less than 10%, preferably not over 5%, by weight of acid monomers, and that the ratio of core weight to the total weight is from 1:4 to 1:100. It was also disclosed that the shell polymer was not permeable to aqueous inorganic base at 20.degree. C., and was not able to form a continuous film at room temperature even in the presence of small amount of coalescing agents because its Tg is considerably higher than 40.degree. C.
To obtain this particular partiole morphology as designed, empolyed was multi-stage emulsion polymerization process where monomers were fed in a preemulsified form under the condition of minimum level of concentration of conventional emulsifier. Because the internal structure of emulsion polymer was controlled by the concentration of the emulsifier in this case, it is difficult to accomplish the polymerization stability and the supression of generation new crop of secondary particles simultaneously. Because the shell-forming monomers are used at least 4 times, preferably 8 times, as such as the core-forming monomers in order to get the concentric core-shell particle structure, the wall of the hollow polymer particles is inevitably thicker than it may be needed.
To solve this problem, in Korea Patent No. 25,024, it was suggested that the copolymerizable surfactant is used as emulsifier to get better water resistance, and that a monomer feed rate, the kinds of polymerizing initiator and the reaction temperature are carefully chosen so as to suppress the formation of abnormal particles during a shell-forming stage and to get a desired particle morphology reproducibly without sacrificing a polymerization stability. It was claimed in the said invention that the concentric core-shell structured latex particles are obtained with using relatively less amount of the shell monomer compared to the method proposed by Kowalski et.al, in U.S. Pat. No. 4,427,836, because it provides higher encapsulation efficiency of the shell polymer on the core particle through suppressing the formation of abnormal particles.
Although the said methods can be used to produce aqueous dispersion of composite latex particles which have the alkali-swellable polymer as core and more hydrophobic polymer as shell, those suffer from the fact that, during drying, particles are collapsed especially when the strength or thickness of polymer wall is not sufficient enough to withstand the contraction force generated by the evaporation of water and the restoration of the swellen core polymer. Such formation of abnormal collapsed particles decreases their hiding efficiency by reduction of the volume of internal void and also restricts paint formulation due to the increases of the oil absorption amount and the binder demand of particles because of unnecessary increase of surface area of particles resulted from a non-spherical particle geometry. To solve this problem, methods employed are to thicken the shell layer of particles by increasing the ratio of shell-forming monomer to core-forming monomer and to prevent the particle collapse by increasing the strength of wall incorporating a proper amount of crosslinking monomer in a mixture of shell-forming monomer, but those are not able to maximize the hiding efficiency per unit weight of particles by unnecessary increase of a wall thickness.